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Photodetachment microscopy

Aim of the research :

In the usual photodetachment experiments carried out in the presence of an electric field, one measures only the total probability of removing the extra electron, as a function of the excitation energy. Photodetachment microscopy, by the use of an electron detector with a high spatial resolution, makes it possible also to observe the spatial distribution of the ejected electron. This is a direct view of the spatial structure of the wave function of an atomic electron, with its node and antinode structure. The axial symmetry of the wave function around the direction of the field makes this structure look like a ring pattern. The photodetachment rings were observed for the first time in 1996 [1], from negative ion Br^-. The phenomenon under study can be considered, from a semi-classical point of view, as interference between two electron waves. The pattern is the figure of interference produced by an electronic dual-wave interferometer, with the negative ion as the electron pointlike source, and the electric field the element that makes trajectories recombine. In the external field, the classical trajectories are those of a motion of constant acceleration, similar to free-fall (cf. fig. 1). Of the emitted electronic wave, a half "falls" directly towards the detector and the other half, initially emitted in the opposite direction, is reflected by the field. The quantitative analysis of the electronic interferograms, essentially by counting the fringes, gives a very precise measurement of the initial kinetic energy of the photodetached electron. This makes it possible, provided one knows the wavelength of the exciting laser, to calculate the detachment threshold of the negative ion under study (cf. fig. 2). This quantity is important, as it is the electron affinity of the neutral atom on which the negative ion was formed [2]. To date, the photodetachment microscopy is the most accurate technique to measure electron affinities (http://en.wikipedia.org/wiki/Electron_affinity_%28data_page%29).

Figure 1 Principle of photodetachment microscopy. (Courtesy of Tobias Kramer)

Figure 2 Definition of electron affinity eA. e is the kinetic energy of the ejected electron, hv is the photon energy.

[1] The Photodetachment Microscope

C. Blondel, C. Delsart and F. Dulieu

Phys. Rev. Lett. 77 3755 (1996)

[2] Effect of a magnetic field in photodetachment microscopy

W. Chaibi, R.J. Pelaez, C. Blondel, C. Drag, and C. Delsart

Eur. Phys. J. D 58, 29 (2010)